Most new aromatics complexes are designed to maximize the yield of benzene and C8 aromatic isomer (para-xylene, meta-xylene, ethylbenzene and ortho-xylene). Para-xylene, meta-xylene and ortho-xylene, are important intermediates which find wide and varied application in chemical syntheses. Para-xylene upon oxidation yields terephthalic acid which is used in the manufacture of synthetic textile fibers and resins. Meta-xylene is used in the manufacture of thermoplastic resins, plasticizers, azo dyes, wood preservers, etc. Ortho-xylene is feedstock for phthalic anhydride production. The distribution of xylene isomers from catalytic reforming and other sources generally does not match that of the sought isomers for chemical intermediates and thus the producer converts the feedstocks to generate more of the sought isomers in the aromatics complexes.
The production of xylenes is practiced commercially in large-scale facilities and is highly competitive. Concerns exist not only about the efficient conversion of feedstock through one or more of isomerization, transalkylation and disproportionation steps to produce xylenes, but also other competitive aspects with respect to such facilities including capital costs and energy costs.
A prior art aromatics complex flow scheme has been disclosed by Meyers in the HANDBOOK OF PETROLEUM REFINING PROCESSES, 2d. Edition in 1997 by McGraw-Hill.
In an aromatics complex, the para-xylene lean stream from para-xylene separation unit is typically routed to the isomerization unit to re-establish the xylene equilibrium and convert ethyl benzene (EB). In vapor phase, the isomerization process with EB dealkylation type catalyst, ethyl benzene is dealkylated to form benzene and xylenes are isomerized to establish equilibrium. On the other hand, the isomerization process with EB isomerization type catalyst converts ethyl benzene in the feed to xylenes via a naphthene intermediate pathway while still reestablishing xylene equilibrium. The performance of the EB isomerization unit is highly sensitive to the ortho-xylene content of the feed with higher ortho-xylene levels leading to higher operating temperatures and therefore higher ring losses. Much of this additional ring loss is from ring opening and cracking (unrecoverable) leading to lower xylene yields in the aromatics complex.
Accordingly, it is desirable to provide an improved process and apparatus for production of para-xylene that can lower the yield losses occurring in the EB isomerization unit to improve para-xylene production. Further, it is desirable to control the amount of ortho-xylene being fed to the EB isomerization unit. Furthermore, other desirable features and characteristics of the present subject matter will become apparent from the subsequent detailed description of the subject matter and the appended claims, taken in conjunction with the accompanying drawings and this background of the subject matter.